Javier Moros, Luisa María Cabalín, and Javier Laserna

UMALaserLab, Department of Analytical Chemistry, University of Málaga 4th Jiménez Fraud, E-29071 Málaga, Spain

Laser-Induced Breakdown Spectroscopy: an Analytical Technique for Sorting of Waste Refractory Materials Used in Steelmaking

Refractories are materials produced from non-metallic minerals that are resistant to high temperature, used predominantly as industrial furnaces linings for elevated temperature materials processing and other applications in which thermomechanical properties are critical. Refractories are indispensable for all high temperatures processes, such as the production of metals, cement, glass and ceramics. Over a great many years recycling tasks on spent refractories was receiving little attention due to the abundance of low cost virgin raw materials and low disposal costs of the, largely inert, materials. However, during the last years, interests in recycling of spent refractories has growing due to many of high quality raw materials are becoming increasingly difficult to come by, and prices are rising.

Upper nozzle

Lower nozzle

Impact brick

Working brick

Safety brick

10 refractory materials used in different activities of the steelmaking process

■ Accurate classification of refractory materials based on their majority component, Al2O3 or MgO, demonstrated

■ MgO-C bricks and MgO tundish masses perfectly distinguished

■ A more fine labeling of Al2O3-bearing materials is needed

■ Decision tree needs to be updated including greater variety of refractory material, at least those “blind samples” that were not included in the initial model

■ After the inclusion into the model of a larger number of samples that better reflect the material variety to be classified, some threshold values, mainly for the Al/Si and Al/Ti intensity ratios, must to be tuned

■ This is how a better classification is foreseen in the future

Detection device AvaSpec-ULS3648-2-USB2 CCD spectrometer Spectral range: 250 nm to 550 nm Temporal settings Delay time = 3.28 µs Integration time = 1.1 ms

Light delivering-&-collecting system

Collimating lens f=20 mm

Laser beam

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ple

Lase

r hea

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Excitation source

Focusing lens f=75.6 mm

Nd:YAG laser Big Sky Ultra 100 Repetition rate (Hz) ≤ 20 Nominal energy (mJ) 1064 nm 100 Laser pulse length (ns) 9.6

The project SYSTEMATIC AND INTEGRAL VALORIZATION OF REFRACTORIES UNDER THE "5R" APPROACH (LIFE5REFRACT) is cofounded by the LIFE financial instrument of the European Community under contract number LIFE17 ENV/ES/000228.

Leader Technical coordinator Partners

https://www.life5refract.eu/es/

Furthermore, reusing recycled refractories has gained interest because of the potential benefits both from an economic (cheaper raw materials, lower treatment costs, reducing costs for landfilling) and environmental (saving virgin resources, reducing wastes and lower energy demand and CO2 emissions compared to virgin materials) point-of-view. This is why technological innovations in recycling plants for refractory wastes as raw materials for the production of new refractories are being focused onto automate the sorting process according to the type of refractory. In this context, a LIBS (Laser-Induced Breakdown spectroscopy) based strategy is being developed to proper sorting of spent refractories.

Note: Analogous refractory materials after operating in the steel manufacture (post-mortem) have been also provided to be assayed

Representative broadband LIBS spectra of some refractory materials

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Al(I) - 308.21 nm

Si(I) – 288.16 nm

Emission intensity (Al at 308.21 nm) Emission intensity (Mg at 280.23 nm)

<1 ≥1

Alumina-based refractory (R#1, R#2, R#3) Spinel (R#8) Nozzle (R#9, R#10)

Emission intensity (Al at 308.21 nm) Emission intensity (Si at 288.15 nm)

Alumina-based refractory (R#1, R#2, R#3)

Spinel (R#8) Nozzle (R#9, R#10)

<15 >15

Zr 587.98 nm 612.78 nm 613.45 nm 614.32 nm 631.30 nm 709.77 nm 716.90 nm

<3sb

Nozzle (R#9)

Zirconia Nozzle (R#10)

>3sb

Emission intensity (Al at 308.21 nm) Emission intensity (CN at 388.18 nm)

<1

Chamotte (R#1)

>1

Alumina-based refractory (R#2, R#3)

Emission intensity (Al at 308.21 nm) Emission intensity (Ti at 498.17 nm)

<3 >3

Alumina (R#2) High-alumina (R#3)

Emission intensity (Al at 308.21 nm) Emission intensity (Si at 288.15 nm)

>5

Nozzle (R#9, R#10)

<5

Spinel (R#8)

LIBS responses from refractory material

Magnesia-based refractory

CN at 388.18 nm

>3sb

Others

<3sb

MgO-C (R#4, R#5, R#6, R#7)

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CN

CN

CN

C2

C2

C2

Chemical composition (% w/w) Sample Principal composition Use Al2O3 MgO Fe2O3 SiO2 TiO2 CaO K2O Na2O ZrO2 C

R#1 Chamotte Safety lining brick 41.0 1.5 54.0 1.7 R#2 Alumina Safety lining brick 69.0 0.1 0.9 29.0 0.6 0.1 0.3 0.1 R#3 High Alumina Safety lining brick 80.5 1.4 13.0 3.1 R#4 Magnesia-Carbon Working lining brick 0.2 96.9 0.3 0.5 1.9 10.0 R#5 Magnesia-Carbon Working lining brick 0.7 96.2 0.5 0.7 1.9 10.0 R#6 Magnesia-Carbon Working lining brick 0.2 97.0 0.3 0.6 1.8 10.0 R#7 Magnesia-Carbon Working lining brick 96.73 0.77 1.14 1.12 10.0 R#8 Alumina-Espinel Impact brick 89.0 8.0 0.2 1.0 0.7 0.4 7.0 R#9 Alumina + Carbon Upper nozzle 60.5 10.5 30.0

R#10 Zirconia + Carbon Lower nozzle 5.5 79.5 15.5

Unseen Sample Predicted group

membership Actual group membership

Unseen Sample Predicted group

membership Actual group membership

Unseen Sample Predicted group

membership Actual group membership

B#01 MgO-C MgO-C B#16 Alumina-carbon nozzle Alumina-carbon nozzle B#31 Spinel High-alumina B#02 Alumina High-alumina B#17 Alumina High-alumina B#32 MgO-C MgO-C B#03 High-alumina High-alumina B#18 High-alumina High-alumina B#33 Spinel Spinel B#04 MgO-C MgO-C B#19 Magnesia-based others Magnesia-based others B#34 Spinel High-alumina B#05 High-alumina High-alumina B#20 High-alumina High-alumina B#35 MgO-C MgO-C B#06 Alumina Chamotte B#21 Alumina Alumina B#36 Alumina-carbon nozzle High-alumina B#07 High-alumina High-alumina B#22 Alumina-carbon nozzle Alumina-carbon nozzle B#37 Spinel Spinel B#08 Magnesia-based others Magnesia-based others B#23 Magnesia-based others Magnesia-based others B#38 Zirconia-carbon nozzle Alumina-carbon nozzle B#09 Alumina High-alumina B#24 MgO-C MgO-C B#39 Magnesia-based others Magnesia-based others B#10 Alumina-carbon nozzle Alumina-carbon nozzle B#25 MgO-C MgO-C B#40 Zirconia-carbon nozzle Zirconia-carbon nozzle B#11 Zirconia-carbon nozzle Zirconia-carbon nozzle B#26 High-alumina High-alumina B#41 Zirconia-carbon nozzle Alumina-carbon nozzle B#12 Alumina-carbon nozzle Alumina-carbon nozzle B#27 Spinel High-alumina B#42 Zirconia-carbon nozzle Zirconia-carbon nozzle B#13 Alumina Alumina B#28 Alumina Chamotte B#43 Zirconia-carbon nozzle Alumina-carbon nozzle B#14 MgO-C MgO-C B#29 High-alumina High-alumina B#44 Zirconia-carbon nozzle Zirconia-carbon nozzle B#15 Alumina High-alumina B#30 Alumina Alumina B#45 Zirconia-carbon nozzle Zirconia-carbon nozzle

Mg(I) - 280.23 nm

—— R#1 —— R#2 —— R#3

—— R#4 —— R#5 —— R#6 —— R#7

Magnesia-based refractories

The number of LIBS analysis of each refractory fragment depends on its size. However, a minimum of 15 laser events per position at 6 different positions on the surface of the material have been performed

Alumina-based refractories

On the blind test set, 32 of 45 samples were correctly classified (≈71%) Samples B#02, B#06, B#09, B#15, B#17 and B#28, while well-identified as alumina-based refractories, are not correctly classified into their specific groups. However, blinds B#02, B#09, B#15 and B#17 are samples different to those used for modeling. LIBS data from samples B#27, B#31, B#34, evidence the presence of C, either inherent in the material or contamination. For this reason, the classification follows the Spinel/Noozle branch instead of instead of continuing for the Alumina–based refractory branch. The same justification is valid for the classification of the sample B#36. Samples B#38, B#41 and B#43, are false positives, which alert of the presence of Zr. A result incorrectly indicating the presence of Zr is preferable to prevent samples containing it from being labeled as free of it.

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Some handicaps for the classification process

Almost the same Mg content (≈96%) but different intensity for its associated emission line

Same C content (10%) and similar intensity for its associated emission signal

Different Al and Si contents but similar intensity for their associated emission lines as a function of samples

Because of refractory material heterogeneity.... Decision-making for labeling is based on the averaged value for intensity ratios of considered emission signals from all the useful LIBS data (a priori 80 spectra -15×6-) gathered from each material

In the case of decisions about the presence of an analyte (CN and Zr) on the basis of 3sb criterion, at least 60% of LIBS spectra from the material must give an affirmative answer

Decisions on the basis of single-shot analysis cannot be considered accurate